What speaker has the best high frequencies?

Well, since size of the tweeter is your criterion, then the "best" would be the three panels in the Kingsound King III electrostatic speaker, of course. (reviewed for Dagogo.com) 

Tweeters

An AMT tweeter is like a bellows it’s surface area is 3-4 x it total
dispersion area that is why it too has a wave guide .The Spatial use a huge AMT to cover midrange on up ,where others use a specific driver just for midrange which ion a $$ speaker is more costly like a Accuton driver, which also use a diamond particle tweeter which is $$ but very accurate. The spatial is very good 
upgrading or requesting a Xover upgrade will bring Huge results most everyone skimps In capacitors resistors , Path audio best made period but $30 each , vs $3 ,and many upgrade capacitors available. Inductors like a Jantzen great quality,value not too costly 
but many use cheap bobbin type or called sledge hammer coil of 
wire around a magnet 🧲 you see this often in many under $10 k 
just to save under $100. Just for the record when ordering ask to 
have Xover upgrade options  humble homemade Hifi capacitor test gives a very good comparison for modt caps.
Spatial uses a Beyma AMT made in Spain which is very good 
other companies use a smaller AMT just for upper frequencies like Monitor audio for  example Their pl500 can go up to 100 k which is class leading ,even their studio can go to 60 kHz which is like a super tweeter. 

Nola Metro Grand Gold 2. Nola uses a 4 inch ribbon tweeter sourced from Raal. They reach high frequencies like I’ve never heard. Very detailed but never harsh

X3-06 EXOTIC T35

But I have heard them also with a solid copper modified baffle.
SEASExotic-T35Tweeters(CopperFaceplate)

The brass terminals were replaced with copper tubes, and the leads crimped and soldered.

1500-25KHz